The present invention relates to a method and an apparatus for forming a shadow mask for a color cathode ray tube (CRT), and more particularly, to such method and apparatus for forming a shadow mask for a square planar (SP) CRT.
A shadow mask is a perforated conducting sheet disposed behind a CRT faceplate having a similar contour as the faceplate, for the purpose of permitting three electron beams to land only on respective color-emitting phosphors deposited on the faceplate. Typically, a conventional spherical shadow mask is formed from a blank by first clamping the blank around its periphery with a clamping means between a pad and a punch. The pad and the punch are shaped with a contour to give the desired mask contour, i.e. allowance is made for springback after forming. The punch is then brought towards said pad stretching the blank to give it the desired mask contour.
Recently, interest has been directed at the "square planar" CRT, such as shown in U.S. application Ser. No. 469,774, filed Feb. 25, 1983, in the name of the present inventor and under common assignment herewith. This application discloses a CRT having different faceplate curvatures along its major and minor axes. This results in a planar edge for the faceplate resulting in a flatter appearance of the tube screen. The shadow mask for such square planar tube also has substantially different curvatures along its major and minor axes. When the above technique for forming shadow masks is used for the square planar tube shadow masks, wrinkles, due to excess material, occur near the ends of the major (long) axis, i.e., along the short sides of the mask. The reason for this is that near the ends of the major axis the curvature is nearly straight in a direction parallel to the minor axis. Since the ends along the short sides are held by curved dies, the mask blank is now curved. When trying to press the curved blank into a straight shape, there is exces material present and hence wrinkles.